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Sep 22, 2013
09/13

by
Mushtaq Loan; Chris Hamer

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A comprehensive analysis of the Symanzik improved anisotropic three-dimensional U(1) lattice gauge theory in the Hamiltonian limit is made. Monte Carlo techniques are used to obtain numerical results for the static potential, ratio of the renormalized and bare anisotropies, the string tension, lowest glueball masses and the mass ratio. Evidence that rotational symmetry is established more accurately for the Symanzik improved anisotropic action is presented. The discretization errors in the...

Source: http://arxiv.org/abs/hep-lat/0307029v5

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Sep 17, 2013
09/13

by
C. J. Hamer

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Energy eigenvalues and order parameters are calculated by exact diagonalization for the transverse Ising model on square lattices of up to 6x6 sites. Finite-size scaling is used to estimate the critical parameters of the model, confirming universality with the three-dimensional classical Ising model. Critical amplitudes are also estimated for both the energy gap and the ground-state energy.

Source: http://arxiv.org/abs/cond-mat/0007063v1

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Sep 18, 2013
09/13

by
C. J. Hamer

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We review the theoretical behaviour of the total and one-particle structure factors at a quantum phase transition for temperature T=0. The predictions are compared with exact or numerical results for the transverse Ising model, the alternating Heisenberg chain, and the bilayer Heisenberg model. At the critical wavevector, the results are generally in accord with theoretical expectations. Away from the critical wavevector, however, different models display quite different behaviours for the...

Source: http://arxiv.org/abs/0809.2857v1

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Sep 23, 2013
09/13

by
C. J. Hamer

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The energy spectrum of the two-magnon bound states in the Heisenberg-Ising antiferromagnet on the square lattice are calculated using series expansion methods. The results confirm an earlier spin-wave prediction of Oguchi and Ishikawa, that the bound states vanish into the continuum before the isotropic Heisenberg limit is reached.

Source: http://arxiv.org/abs/0904.4085v1

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Sep 18, 2013
09/13

by
A. Collins; C. J. Hamer

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The S=1/2 Heisenberg bilayer spin model at zero temperature is studied in the dimerized phase using analytic triplet-wave expansions and dimer series expansions. The occurrence of two-triplon bound states in the S=0 and S=1 channels, and antibound states in the S=2 channel, is predicted by the triplet-wave theory, and confirmed by series expansions. All bound states are found to vanish at or before the critical coupling separating the dimerized phase from the Neel phase. The critical behaviour...

Source: http://arxiv.org/abs/0806.0694v1

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Jul 20, 2013
07/13

by
J. Oitmaa; C. J. Hamer

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We calculate ground state properties (energy, magnetization, susceptibility) and one-particle spectra for the $S = 1$ Heisenberg antiferromagnet with easy-axis or easy-plane single site anisotropy, on the square lattice. Series expansions are used, in each of three phases of the system, to obtain systematic and accurate results. The location of the quantum phase transition in the easy-plane sector is determined. The results are compared with spin-wave theory.

Source: http://arxiv.org/abs/0804.1171v1

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Sep 19, 2013
09/13

by
J. Oitmaa; C. J. Hamer

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We use high-temperature series expansions to obtain thermodynamic properties of the quantum compass model, and to investigate the phase transition on the square and simple cubic lattices. On the square lattice we obtain evidence for a phase transition, consistent with recent Monte Carlo results. On the simple cubic lattice the same procedure provides no sign of a transition, and we conjecture that there is no ?nite temperature transition in this case.

Source: http://arxiv.org/abs/1010.3804v1

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Sep 20, 2013
09/13

by
W. Zheng; C. J. Hamer

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The two-hole excitation spectrum of the t-J ladder at half-filling is studied using linked-cluster series expansion methods. A rich spectrum of bound states emerges, particularly at small $t/J$. Their dispersion relations and coherence lengths are computed, along with the threshold behaviour as the bound states merge into the continuum. A class of 4-hole bound states is also studied, leading to the conclusion that phase separation occurs for $t/J \lesssim 0.5$, in agreement with other studies.

Source: http://arxiv.org/abs/cond-mat/0204514v1

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Sep 20, 2013
09/13

by
Zheng Weihong; C. J. Hamer

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Spin-wave perturbation theory for the Heisenberg antiferromagnet at zero temperature is used to compute the finite-lattice corrections to the ground state energy, the staggered magnetization and the energy gap. The dispersion relation, the spin-wave velocity and the bulk ground state energy to order $O(1/S^2)$ are also computed for the square lattice. The results agree very well with the predictions of Neuberger and Ziman and Fisher.

Source: http://arxiv.org/abs/cond-mat/9212020v1

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Sep 19, 2013
09/13

by
P. Sriganesh; R. Bursill; C. J. Hamer

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A new finite lattice calculation of the low lying bound state energies in the massive Schwinger model is presented, using a Hamiltonian lattice formulation. The results are compared with recent analytic series calculations in the low mass limit, and with a new higher order non-relativistic series which we calculate for the high mass limit. The results are generally in good agreement with these series predictions, and also with recent calculations by light cone and related techniques.

Source: http://arxiv.org/abs/hep-lat/9911021v2

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Sep 23, 2013
09/13

by
C. J. Hamer; Zheng Weihong; J. Oitmaa

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We study one and two hole properties of the t-J model at half-filling on the square lattice using series expansion methods at T=0. The dispersion curve for one hole excitations is calculated and found to be qualitatively similar to that obtained by other methods, but the bandwidth for small $t/J$ is some 20% larger than given previously. We also obtain the binding energy and dispersion relation for two hole bound states. The lowest bound state as $t/J$ increases is found to be first d-wave, and...

Source: http://arxiv.org/abs/cond-mat/9806367v2

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Sep 22, 2013
09/13

by
Onofre Rojas; C. J. Hamer; J. Oitmaa

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We study a frustrated 3D antiferromagnet of stacked $J_1 - J_2$ layers. The intermediate 'quantum spin liquid' phase, present in the 2D case, narrows with increasing interlayer coupling and vanishes at a triple point. Beyond this there is a direct first-order transition from N{\' e}el to columnar order. Possible applications to real materials are discussed.

Source: http://arxiv.org/abs/1103.1679v1

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Sep 19, 2013
09/13

by
Mushtaq Loan; Michael Brunner; Chris Hamer

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A numerical study of low-lying glueball masses of compact U(1) lattice gauge theory in (2+1) dimensions is performed using Standard Path integral Monte Carlo techniques. The masses are extracted, at fixed (low) temperature, from simulations on anisotropic lattices, with temporal lattice spacing much smaller than the spatial ones. Convincing evidence of the scaling behaviour in the antisymmetric mass gap is observed over the range $1.4

Source: http://arxiv.org/abs/hep-lat/0209161v1

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Jul 20, 2013
07/13

by
Zheng Weihong; J. Oitmaa; C. J. Hamer

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The Shastry-Sutherland model, which provides a representation of the magnetic properties of SrCu$_2$(BO$_3$)$_2$, has been studied by series expansion methods at T=0. Our results support the existence of an intermediate phase between the N\'eel long-range ordered phase and the short-range dimer phase. They provide strong evidence against the existence of helical order in the intermediate phase, and and somewhat less strong evidence against a plaquette-singlet phase. The nature of the...

Source: http://arxiv.org/abs/cond-mat/0107019v1

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Sep 18, 2013
09/13

by
A. Collins; C. J. Hamer; Zheng Weihong

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An alternative triplet-wave expansion formalism for dimerized spin systems is presented, a modification of the 'bond operator' formalism of Sachdev and Bhatt. Projection operators are used to confine the system to the physical subspace, rather than constraint equations. The method is illustrated for the case of the alternating Heisenberg chain, and comparisons are made with the results of dimer series expansions and exact diagonalization. Some discussion is included of the phenomenon of...

Source: http://arxiv.org/abs/cond-mat/0605724v4

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Sep 22, 2013
09/13

by
Zheng Weihong; J. Oitmaa; C. J. Hamer

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We study antiferromagnetic, $S=1/2$ Heisenberg models with nearest and second neighbor interactions on the one-fifth depleted square lattice which describes the spin degrees of freedom in the spin-gap system CaV$_4$O$_9$. The meta-plaquette expansion for the triplet excitation spectrum is extended to fifth order, and the results are compared with experimental data on CaV$_4$O$_9$. We attempt to locate the phase boundary between magnetically ordered and gapped phases.

Source: http://arxiv.org/abs/cond-mat/9807249v1

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Sep 19, 2013
09/13

by
Mushtaq Loan; Tim Byrnes; Chris Hamer

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The Numerical calculations for tadpole-improved U(1) lattice gauge theory in three-dimensions on anisotropic lattices have been performed using standard path integral Monte Carlo techniques. Using average plaquette tadpole renormalization scheme, simulations were done with temporal lattice spacings much smaller than the spatial ones and results were obtained for the string tension, the renormalized anisotropy and scalar glueball masses. We find, by comparing the `regular' and `sideways'...

Source: http://arxiv.org/abs/hep-lat/0303011v3

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Sep 23, 2013
09/13

by
Anders W. Sandvik; Chris J. Hamer

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We present high-precision quantum Monte Carlo results for the S=1/2 XY model on a two-dimensional square lattice, in the ground state as well as at finite temperature. The energy, the spin stiffness, the magnetization, and the susceptibility are calculated and extrapolated to the thermodynamic limit. For the ground state, we test a variety of finite-size scaling predictions of effective Lagrangian theory and find good agreement and consistency between the finite-size corrections for different...

Source: http://arxiv.org/abs/cond-mat/9904220v1

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Sep 18, 2013
09/13

by
Zheng Weihong; C. J. Hamer; J. Oitmaa

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We use a variety of series expansion methods at both zero and finite temperature to study an antiferromagnetic Heisenberg spin model proposed recently by Miyahara and Ueda for the quasi two-dimensional material SrCu$_2$(BO$_3$)$_2$. We confirm that this model exhibits a first-order quantum phase transition at T=0 between a gapped dimer phase and a gapless N\'eel phase when the ratio $x=J'/J$ of nearest and next-nearest neighbour interactions is varied, and locate the transition at...

Source: http://arxiv.org/abs/cond-mat/9811030v1

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Sep 23, 2013
09/13

by
J. Oitmaa; C. J. Hamer; Zheng Weihong

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Series expansions at T=0 are used to study properties of the half-filled $t-J$ ladder doped with one or two holes, and at quarter filling. Dispersion curves are obtained for one-hole symmetric and antisymmetric (bonding and antibonding) excitations and for the two-hole bound state. The line in the phase diagram which separates bound and unbound states is determined. For quarter filling we compute the ground state energy and estimate the location of the phase separation line. Comparisons with...

Source: http://arxiv.org/abs/cond-mat/9909280v2

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Sep 20, 2013
09/13

by
D. SchÃ¼tte; Zheng Weihong; C. J. Hamer

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The coupled cluster or exp S form of the eigenvalue problem for lattice Hamiltonian QCD (without quarks) is investigated. A new construction prescription is given for the calculation of the relevant coupled cluster matrix elements with respect to an orthogonal and independent loop space basis. The method avoids the explicit introduction of gauge group coupling coefficients by mapping the eigenvalue problem onto a suitable set of character functions, which allows a simplified procedure. Using...

Source: http://arxiv.org/abs/hep-lat/9603026v1

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Sep 18, 2013
09/13

by
C. J. Hamer; J. Oitmaa; Zheng Weihong

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Strong-coupling series expansions are calculated for the Hamiltonian version of compact lattice electrodynamics in (2+1) dimensions, with 4-component fermions. Series are calculated for the ground-state energy per site, the chiral condensate, and the masses of `glueball' and positronium states. Comparisons are made with results obtained by other techniques.

Source: http://arxiv.org/abs/hep-lat/9710008v1

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Sep 24, 2013
09/13

by
Weihong Zheng; J. Oitmaa; C. J. Hamer

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We have extended our previous series studies of quantum antiferromagnets at zero temperature by computing the one-magnon dispersion curves and various structure factors for the linear chain, square and simple cubic lattices. Many of these results are new; others are a substantial extension of previous work. These results are directly comparable with neutron scattering experiments and we make such comparisons where possible.

Source: http://arxiv.org/abs/cond-mat/0412184v2

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Sep 19, 2013
09/13

by
Mushtaq Loan; Michael Brunner; Chris Hamer

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Compact U(1) lattice gauge theory in (2+1) dimensions is studied on anisotropic lattices using Standard Path Integral Monte Carlo techniques. We extract the static quark potential and the string tension from 1.0

Source: http://arxiv.org/abs/hep-lat/0208047v1

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Sep 18, 2013
09/13

by
Weihong Zheng; Chris Hamer; Rajiv Singh

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We calculate the excitation spectrum and spectral weights of the alternating antiferromagnetic-ferromagnetic spin-half Heisenberg chain with exchange couplings $J$ and $-|\lambda|J$ as a power series in $\lambda$. For small $|\lambda|$, the gapped one-particle spectrum has a maximum at $k=0$ and there is a rich structure of bound (and anti-bound) states below (and above) the 2-particle continuum. As $|\lambda|$ is increased past unity the spectrum crosses over to the Haldane regime, where the...

Source: http://arxiv.org/abs/cond-mat/0605370v1

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Jun 28, 2018
06/18

by
Quentin SalomÃ©; Philippe SalomÃ©; FranÃ§oise Combes; Stephen Hamer; Ian Heywood

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Centaurus A is the most nearby powerful AGN, widely studied at all wavelengths. Molecular gas has been found in the halo at a distance of ~20 kpc from the galaxy centre, associated with HI shells. The molecular gas lies inside some IR and UV bright star-forming filaments that have recently been observed in the direction of the radio jets. These archival data show that there is dust and very weak star formation on scales of hundreds of parsecs. On top of analysing combined archival data, we have...

Topics: Astrophysics, Astrophysics of Galaxies

Source: http://arxiv.org/abs/1511.04310

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Jun 29, 2018
06/18

by
Quentin SalomÃ©; Philippe SalomÃ©; FranÃ§oise Combes; Stephen Hamer

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NGC 5128 (Centaurus A) is one of the best example to study AGN-feedback in the local Universe. At 13.5 kpc from the galaxy, optical filaments with recent star formation are lying along the radio-jet direction. We used the Atacama Pathfinder EXperiment (APEX) to map the CO(2-1) emission all along the filaments structure. Molecular gas mass of 8.2x10^7 Msun was found over the 4.2 kpc-structure which represents about 3% of the total gas mass of the NGC 5128 cold gas content. Two dusty mostly...

Topics: Astrophysics, Astrophysics of Galaxies

Source: http://arxiv.org/abs/1605.05986

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Jun 30, 2018
06/18

by
Stephen Hamer; Philippe SalomÃ©; Francoise Combes; Quentin SalomÃ©

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Evidence of AGN interaction with the IGM is observed in some galaxies and many cool core clusters. Radio jets are suspected to dig large cavities into the surrounding gas. In most cases, very large optical filaments are seen around the central galaxy. The origin of these filaments is still not understood. Star-forming regions are sometimes observed inside the filaments and are interpreted as evidence of positive feedback. Cen A is a nearby galaxy with huge optical filaments aligned with the AGN...

Topics: Astrophysics of Galaxies, Astrophysics

Source: http://arxiv.org/abs/1409.7700

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Sep 19, 2013
09/13

by
Robert J. Bursill; Ross H. McKenzie; Chris J. Hamer

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Using a new version of the density-matrix renormalization group we determine the phase diagram of a model of an antiferromagnetic Heisenberg spin chain where the spins interact with quantum phonons. A quantum phase transition from a gapless spin-fluid state to a gapped dimerized phase occurs at a non-zero value of the spin-phonon coupling. The transition is in the same universality class as that of a frustrated spin chain, which the model maps to in the anti-adiabatic limit. We argue that...

Source: http://arxiv.org/abs/cond-mat/9812409v2

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Sep 22, 2013
09/13

by
Chris J. Hamer; Weihong Zheng; Rajiv R. P. Singh

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We develop a linked cluster method to calculate the spectral weights of many-particle excitations at zero temperature. The dynamical structure factor is expressed as a sum of exclusive structure factors, each representing contributions from a given set of excited states. A linked cluster technique to obtain high order series expansions for these quantities is discussed. We apply these methods to the alternating Heisenberg chain around the dimerized limit ($\lambda=0$), where complete wavevector...

Source: http://arxiv.org/abs/cond-mat/0307517v1

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Sep 18, 2013
09/13

by
Chris J. Hamer; Robert J. Bursill; Maria Samaras

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A ``forward walking'' Quantum Monte Carlo (QMC) algorithm has been developed to calculate correlation functions for the Hamiltonian lattice formulation of U(1) Yang-Mills theory in (2+1) dimensions. It is shown that Wilson loops can be calculated with high accuracy. Creutz ratios are used to determine the string tension, which agrees with results from other approaches. Timelike correlations are used to estimate the mass gaps, which agree with series expansion results in the strong coupling...

Source: http://arxiv.org/abs/hep-lat/0002001v1

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Sep 23, 2013
09/13

by
Ross H. McKenzie; C. J. Hamer; D. W. Murray

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The Holstein model of spinless fermions interacting with dispersionless phonons in one dimension is studied by a Green's function Monte Carlo technique. The ground state energy, first fermionic excited state, density wave correlations, and mean lattice displacement are calculated for lattices of up to 16 sites, for one fermion per two sites, i.e., a half-filled band. Results are obtained for values of the fermion hopping parameter of $t=0.1 \omega$, $\omega$, and $10 \omega$ where $\omega$ is...

Source: http://arxiv.org/abs/cond-mat/9512046v2

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Sep 22, 2013
09/13

by
A. Fabricio Albuquerque; Chris J. Hamer; Jaan Oitmaa

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We investigate the zero-temperature phase diagram of the one-dimensional S=1 Heisenberg antiferromagnet with single-ion anisotropy. By employing high-order series expansions and quantum Monte Carlo simulations we obtain accurate estimates for the critical points separating different phases in the quantum phase diagram. Additionally, excitation spectra and gaps are obtained.

Source: http://arxiv.org/abs/0811.0646v2

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Sep 21, 2013
09/13

by
Martin P. Gelfand; Zheng Weihong; C. J. Hamer; J. Oitmaa

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Spin-S bilayer Heisenberg models (nearest-neighbor square lattice antiferromagnets in each layer, with antiferromagnetic interlayer couplings) are treated using dimer mean-field theory for general S and high-order expansions about the dimer limit for S=1, 3/2,...,4. We suggest that the transition between the dimer phase at weak intraplane coupling and the Neel phase at strong intraplane coupling is continuous for all S, contrary to a recent suggestion based on Schwinger boson mean-field theory....

Source: http://arxiv.org/abs/cond-mat/9705201v1

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Sep 20, 2013
09/13

by
C. J. Hamer; J. Oitmaa; Weihong Zheng; Ross H. McKenzie

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We investigate the critical behaviour of the spectral weight of a single quasiparticle, one of the key observables in experiment, for the particular case of the transverse Ising model.Series expansions are calculated for the linear chain and the square and simple cubic lattices. For the chain model, a conjectured exact result is discovered. For the square and simple cubic lattices, series analyses are used to estimate the critical exponents. The results agree with the general predictions of...

Source: http://arxiv.org/abs/cond-mat/0607589v1

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Sep 23, 2013
09/13

by
A. Weisse; R. J. Bursill; C. J. Hamer; Zheng Weihong

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The phase diagram of the two-leg t-Jz ladder is explored, using the density matrix renormalization group method. Results are obtained for energy gaps, electron density profiles and correlation functions for the half-filled and quarter-filled cases. The effective Lagrangian velocity parameter is shown to vanish at half-filling. The behaviour of the one-hole gap in the Nagaoka limit is investigated, and found to disagree with theoretical predictions. A tentative phase diagram is presented, which...

Source: http://arxiv.org/abs/cond-mat/0511528v1

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Sep 19, 2013
09/13

by
Mushtaq Loan; Michael Brunner; Clare Sloggett; Chris Hamer

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Path Integral Monte Carlo simulations have been performed for U(1) lattice gauge theory in (2+1) dimensions on anisotropic lattices. We extractthe static quark potential, the string tension and the low-lying "glueball" spectrum.The Euclidean string tension and mass gap decrease exponentially at weakcoupling in excellent agreement with the predictions of Polyakov and G{\" o}pfert and Mack, but their magnitudes are five times bigger than predicted. Extrapolations are made to the...

Source: http://arxiv.org/abs/hep-lat/0209159v5

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Sep 20, 2013
09/13

by
R. J. Bursill; R. H. McKenzie; C. J. Hamer

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The one-dimensional Holstein model of spinless fermions interacting with dispersionless phonons is studied using a new variant of the density matrix renormalisation group. By examining various low-energy excitations of finite chains, the metal-insulator phase boundary is determined precisely and agrees with the predictions of strong coupling theory in the anti-adiabatic regime and is consistent with renormalisation group arguments in the adiabatic regime. The Luttinger liquid parameters,...

Source: http://arxiv.org/abs/cond-mat/9802215v2

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Sep 17, 2013
09/13

by
C. J. Hamer; M. Samaras; R. J. Bursill

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A 'forward walking' Green's Function Monte Carlo algorithm is used to obtain expectation values for SU(3) lattice Yang-Mills theory in (3+1) dimensions. The ground state energy and Wilson loops are calculated, and the finite-size scaling behaviour is explored. Crude estimates of the string tension are derived, which agree with previous results at intermediate couplings; but more accurate results for larger loops will be required to establish scaling behaviour at weak coupling.

Source: http://arxiv.org/abs/hep-lat/0007031v1

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Sep 23, 2013
09/13

by
C. J. Hamer; M. Sheppeard; Zheng Weihong; D. Schutte

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The Green's Function Monte Carlo method of Chin et al is applied to SU(2) Yang-Mills theory in (2+1)D. Accurate measurements are obtained for the ground-state energy and mean plaquette value, and for various Wilson loops. The results are compared with series expansions and coupled cluster estimates, and with the Euclidean Monte Carlo results of Teper. A striking demonstration of universality between the Hamiltonian and Euclidean formulations is obtained.

Source: http://arxiv.org/abs/hep-th/9511179v1

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Sep 18, 2013
09/13

by
C. J. Hamer; M. Samaras; R. J. Bursill

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A `forward walking' Green's Function Monte Carlo algorithm is used to obtain expectation values for SU(3) lattice Yang-Mills theory in (3+1) dimensions. The ground state energy and Wilson loops are calculated, and the finite-size scaling behaviour is explored. Crude estimates of the string tension are derived, which agree with previous results at intermediate couplings; but more accurate results for larger loops will be required to establish scaling behaviour at weak coupling.

Source: http://arxiv.org/abs/hep-lat/0005009v1

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Sep 18, 2013
09/13

by
T. Byrnes; P. Sriganesh; R. J. Bursill; C. J. Hamer

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The massive Schwinger model is studied, using a density matrix renormalisation group approach to the staggered lattice Hamiltonian version of the model. Lattice sizes up to 256 sites are calculated, and the estimates in the continuum limit are almost two orders of magnitude more accurate than previous calculations. Coleman's picture of `half-asymptotic' particles at background field theta = pi is confirmed. The predicted phase transition at finite fermion mass (m/g) is accurately located, and...

Source: http://arxiv.org/abs/hep-lat/0202014v1

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Sep 22, 2013
09/13

by
T. Byrnes; P. Sriganesh; R. J. Bursill; C. J. Hamer

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The massive Schwinger model is studied, using a density matrix renormalization group approach to the staggered lattice Hamiltonian version of the model. Lattice sizes up to 256 sites are calculated, and the estimates in the continuum limit are almost two orders of magnitude more accurate than previous calculations. Coleman's picture of `half-asymptotic' particles at background field (theta = pi) is confirmed. The predicted phase transition at finite fermion mass (m/g) is accurately located, and...

Source: http://arxiv.org/abs/hep-lat/0201007v1

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Sep 17, 2013
09/13

by
Zheng Weihong; J. Oitmaa; C. J. Hamer; R. J. Bursill

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The Hubbard model on a two-leg ladder structure has been studied by a combination of series expansions at T=0 and the density-matrix renormalization group. We report results for the ground state energy $E_0$ and spin-gap $\Delta_s$ at half-filling, as well as dispersion curves for one and two-hole excitations. For small $U$ both $E_0$ and $\Delta_s$ show a dramatic drop near $t/t_{\perp}\sim 0.5$, which becomes more gradual for larger $U$. This represents a crossover from a "band...

Source: http://arxiv.org/abs/cond-mat/0007388v1

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Jun 30, 2018
06/18

by
E. O'Sullivan; F. Combes; S. Hamer; P. SalomÃ©; A. Babul; S. Raychaudhury

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We present IRAM 30m telescope observations of the CO(1-0) and (2-1) lines in a sample of 11 group-dominant elliptical galaxies selected from the CLoGS nearby groups sample. Our observations confirm the presence of molecular gas in 4 of the 11 galaxies at >4 sigma significance, and combining these with data from the literature we find a detection rate of 43+-14%, comparable to the detection rate for nearby radio galaxies, suggesting that group-dominant ellipticals may be more likely to...

Topics: Astrophysics of Galaxies, Astrophysics

Source: http://arxiv.org/abs/1408.7106

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Sep 23, 2013
09/13

by
Rajiv R. P. Singh; Zheng Weihong; C. J. Hamer; J. Oitmaa

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Ground state energies for plaquette and dimer order in the $J_1-J_2$ square-lattice spin-half Heisenberg model are compared using series expansion methods. We find that these energies are remarkably close to each other at intermediate values of $J_2/J_1$, where the model is believed to have a quantum disordered ground state. They join smoothly with those obtained from the Ising expansions for the 2-sublattice N\'eel-state at $J_2/J_1 \approx 0.4$, suggesting a second order transition from a...

Source: http://arxiv.org/abs/cond-mat/9904064v1

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49

Sep 19, 2013
09/13

by
Weihong Zheng; C. J. Hamer; R. R. P. Singh; Simon Trebst; Hartmut Monien

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We develop strong-coupling series expansion methods to study two-particle spectra of quantum lattice models. At the heart of the method lies the calculation of an effective Hamiltonian in the two-particle subspace. We explicitly consider an orthogonality transformation to generate this block diagonalization, and find that maintaining orthogonality is crucial for systems where the ground state and the two-particle subspace are characterized by identical quantum numbers. We discuss the solution...

Source: http://arxiv.org/abs/cond-mat/0010354v1

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68

Sep 17, 2013
09/13

by
S. Trebst; H. Monien; C. J. Hamer; Z. Weihong; R. R. P. Singh

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We present a new linked cluster expansion for calculating properties of multiparticle excitation spectra to high orders. We use it to obtain the two-particle spectra for systems of coupled spin-half dimers. We find that even for weakly coupled dimers the spectrum is very rich, consisting of many bound states. The number of bound states depends on both geometry of coupling and frustration. Many of the bound states can only be seen by going to sufficiently high orders in the perturbation theory,...

Source: http://arxiv.org/abs/cond-mat/0007192v1

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35

Sep 20, 2013
09/13

by
M. P. Gelfand; Z. Weihong; R. R. P. Singh; J. Oitmaa; C. J. Hamer

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We have carried out a wide range of calculations for the $S=1/2$ Heisenberg model with nearest- and second-neighbor interactions on a two-dimensional lattice which describes the geometry of the vanadium ions in the spin-gap system CaV$_4$O$_9$. The methods used were convergent high-order perturbation expansions (``Ising'' and ``Plaquette'' expansions at $T=0$, as well as high-temperature expansions) for quantities such as the uniform susceptibility, sublattice magnetization, and triplet...

Source: http://arxiv.org/abs/cond-mat/9603025v1

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23

Sep 19, 2013
09/13

by
R. R. P. Singh; W. Zheng; J. Oitmaa; O. P. Sushkov; C. J. Hamer

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The large $J_2$ limit of the square-lattice $J_1-J_2$ Heisenberg antiferromagnet is a classic example of order by disorder where quantum fluctuations select a collinear ground state. Here, we use series expansion methods and a meanfield spin-wave theory to study the excitation spectra in this phase and look for a finite temperature Ising-like transition, corresponding to a broken symmetry of the square-lattice, as first proposed by Chandra et al. (Phys. Rev. Lett. 64, 88 (1990)). We find that...

Source: http://arxiv.org/abs/cond-mat/0303075v1